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Electrical Protection of Induction Motors
Wednesday, January 30, 2019

Three-phase induction motors have a wide variety of applications in commercial and industrial settings. These include heat pumps, air conditioners, compressors and industrial machinery. Given their extensive range of applications, it’s critical to adequately protect 3-phase induction motors. Operating these kinds of motors requires protection against several different events and conditions. In this post, we’ll help you understand those.

Fault Protection

Electrical fault can occur if electric current is abnormally high. Generally, fault currents result from short circuits, line-to-line fault, and line-to-ground fault. In case of motor, fault current can occur if the motor fails to start because of a locked rotor. The duration of fault current will be very short if the motor is able to start normally. However, it’ll extend if motor doesn’t start.

Circuit breakers with “magnetic tripping system” are used to accomplish fault protection. Solenoid coil in these circuit breakers induces a powerful magnetic which causes the motor’s contacts to open. Thus the electric supply to the motor is interrupted. If fault is not interrupted, it can cause permanent damage to the motor.

Overload Protection

When the motor’s current is above its rated value, it is called overload. However, unlike fault current overloads, this current may be a few amperes greater than the motor’s rated current. Nevertheless, motors must be protected against overload as it can cause permanent damage. Electronic protection or thermal protection devices can be used to accomplish overload protection.
  • Typically, thermal protection is based on the motor’s contacts. When the current exceeds the motor’s rated current, a thermal protection device interrupts the circuit.
  • The same result is achieved electronic protection, but through the use and measurement of contactor.
There’s a time delay in overload protection. The reason is that short-durations of over-current conditions are normal in some operations and the protector should allow it.

Over-Voltage/Under-Voltage Protection

Electric motors must be operated within a certain voltage range. If the operating voltage is outside that specific range, problems can occur shortening the motor’s service life. In case of under-voltage, the motor draws more current to maintain the required torque. This damages motor and causes a higher inrush current. In case of over-voltage, the ferromagnetic cores of the motor become saturated, increasing the line current.

Normally, motors have a built-in protection circuit which disconnects the equipment if the voltage exceeds or falls below the specified voltage range.

Phase Imbalance/Phase Loss Protection

If one of the three phases of an induction motor is lost, it can still continue operating. However, when the motor operates without all three phases, it not only puts more mechanical stress on equipment but also increases the current in the other two phases. The condition may cause the motor to fail.

An auxiliary circuit is used to protect the motor from phase imbalance and loss. Phase imbalance should typically not be greater than 2 percent to ensure optimal performance.

For more information, feel free to contact us at Midwest Equipment! We'll be happy to answer all your questions.


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